When components in high frequency circuits are separated by relatively large distances as compared with the frequency at which the circuit is to be used, signals are typically transferred between such components by means of transmission lines of one kind or another. Two such transmission lines that are often used in modern high frequency microcircuits on patterned printed circuit boards and in hybrid circuits are the microstrip transmission line and the stripline transmission line. A microstrip transmission line is formed by separating a metal conductive strip from a parallel conductive ground plane by means of a dielectric layer. A stripline transmission line is constructed similar to that of a microstrip transmission line but has two conductive ground planes, one on each side of the metal conductive strip. The conductive ground planes are separated from the metal conductive strip by dielectrics.
Designing and fabricating microstrip transmission lines and stripline transmission lines for minimum signal loss is of great importance for these devices. In some cases a layer of gold has been included on the metal conductive layer and the ground plane(s) to reduce resistive losses. Also, low loss dielectrics are often used to reduce leakage currents between the metal conductive layer and the ground plane(s).
A loss mechanism which has been found to exist in stripline transmission lines and which can result in a substantial loss of signal strength is caused by the excitation of parallel plate mode signals. The parallel plate mode occurs between the two ground planes in stripline transmission lines when one of the ground planes is capable of attaining a potential which differs from that of the other ground plane.